The present invention relates to devices and methods for illuminating a multichannel linear light modulator array with a linear array of multielement laser diodes.
In the present art a method is known for optically gathering laser light from a multielement laser diode array and imaging it onto a linear light modulator in such a manner that the image of each of the elements or facets of the laser array is superimposed at the linear light modulator. Such systems, as discussed in U.S. Pat. No. 5,521,748 to Sarraf, are used to generate images at high speed during recordation on a light sensitive sheet or other medium. In practical systems, the need for improved optics to improve efficiency and reduce size has become evident.
Another example is found in U.S. Pat. No. 5,517,359 to Gelbart, an object of which is not only to collect and direct most of the light from the laser facets onto the modulator, but to illuminate the entire modulator with the image of each laser facet so as to increase the reliability of the system, should any one emitter fail.
The design is such that, while there is reliance on very wide area laser diodes, the LaGrange product is conserved. The method of design described in U.S. Pat. No. 5,517,359, however, requires high magnification in the objective lens. The image of each laser facet must be enlarged to the size of the modulator. Because of this requirement for high magnification, the design according to the method described in U.S. Pat. No. 5,517,359 many times leads to longer than desired optical paths from the laser to the modulator. However, in the current state of the art redundancy is no longer a prime factor in the need to conserve LaGrange product. There is therefore a need for a compact, optically efficient apparatus that enables a linear array of light sources to illuminate a target area through a linear array of modulators.
A set of optical systems are described wherein the optical path in any given example is much shorter than heretofore achieved but where the modulator is entirely illuminated in all its segments. In one example a combined light bundle emerges through a minimum angular spread and can therefore match a spatial light modulator having a small angular acceptance. In another example the light bundle is differentially converged on the fast and slow axes of the source. In both examples, the sources, such as laser diodes, individually illuminate modulators and target areas because the ray beams, from each laser, are first angled in accordance with the relative lateral spacing of the targets and then directed through individual microlenses lens system in combination with a field lens system to arrive at the modulator with chosen shape factors, angles and areas.
The degrees of freedom afforded by this design are used to increase the dimensions of the laser facet spacings and the dimensions of the microlenses used to collect and focus the laser light. This makes the laser array more able to dissipate thermal energy, and easier to drive to higher powers. It also enables the microlens to be more efficient and easier to assemble. In addition, devices in accordance with the invention may make use of the fast and slow axes of the ray packages to provide contiguous images along the elements of a modulator array, and focus points in the orthogonal direction. Furthermore, the sum of the emittances from all the sources combined can be matched to the acceptance of the modulator for best optical efficiency.
Apparatus for imaging light from a laser diode array onto a multichannel linear light modulator includes in one example, one or more broad emitting area laser diode arrays having multiple emitters operating in parallel to illuminate the linear light modulator. In an arrangement where one laser diode array is used, a microlens array is positioned close to the laser diode so as to project each of the laser emitter facets onto a successive segment of the linear light modulator, with the images or parallel beams from each emitter sequentially arranged along the length of the linear light modulator, the light from each emitter covering a successive portion of the length of the modulator. The array of microlenses has a pitch equal to or greater than the pitch of the emitting laser facets and equal to or less than the center-to-center distance of the illuminated segments of the modulator. The focal length of each element of the microlens array is slightly less than the distance from the laser emitters to an axial distance where the light beams from adjacent emitters start to overlap.
Between the laser array and the microlens array, cylindrical lens means are provided, for example by a long microcylinder, for changing the divergence of the light from the emitters in a direction perpendicular to the longer dimension of the emitting laser facets. Typically the power contributed by the long microcylinder is sufficient to convert the wider divergence of the beams in the narrow or fast axis of the laser facets to become close to the divergence of the beams in the long or slow axis. Such an arrangement results in an optical design in which all channels appear to be operating on-axis, and they each consequently achieve diffraction limited imagery in both the X and Y directions.
In another example, the pitch of the modulator elements is greater than that of the laser elements, and the ray bundles are divergent but also imaged to approximate focus points in both the fast and slow axes. This enables the admittance of the modulator elements to be matched to the emittance of the lasers for best optical efficiency.
In yet another example two laser diode bars are orthogonally disposed with collimators and microlens arrays to direct multiple beams toward an optical cube, which combines both sets in common parallel paths toward a linear modulator. Using the polarization property of the laser a polarization rotator in one of the orthogonal beam sets assures that fast and slow axis orientations from both beam sets are superimposed at the modulator.